The prior art contains several soil moisture sensor designs, each of which attempts to measure soil moisture by electronic means based on the capacitance effect between sensor plates.
A variation of these designs is given in U.S. Pat. No. 4,850,386 to Bireley, which discloses two electrodes and a detector circuit connected to the electrodes to measure the impedance between them. At least one of the electrodes has a relatively thick dielectric coating, so that the sensor reactance is also relatively high. The water droplets in the soil form paths to the dielectric coating between the electrodes, which are preferably situated relatively closely together, to reduce the resistance and minimize the capacitative effect. The impedance between the electrodes varies with the moisture level of the soil, but since the design features a small capacitance, the measurement is not as sensitive to changes in moisture.
In U.S. Pat. No. 4,837,499 to Scherer there is disclosed a soil moisture sensor having a pair of concentrically disposed cylindrical conductors separated by a fibrous material, to form a capacitor whose resistance varies with the moisture level.
In U.S. Pat. No. 4,683,904 to Iltis, there is disclosed a moisture Sensor having an oscillator which changes its frequency as the capacitance between a pair of plates changes due to the moisture between them. The sensor responds to soil moisture changes as a function of the change in the dielectric constant.
In U.S. Pat. No. 4,540,936 to Walsh, there is disclosed a soil moisture sensor provided as a pair of coaxial tubes forming a coaxial capacitor, which can be inserted into the soil. The coaxial capacitor is connected in a Wien bridge circuit for measuring the impedance by application of a variable frequency oscillator and frequency detector, and once the resonant frequency is detected, the capacitance can be derived along with the dielectic constant and soil moisture.
U.S. Pat. No. 3,771,548 to Rauchwerger discloses a variable capacitor provided as two plates between which the moisture is directly proportional to the capacitance between them. An oscillator is used to develop a voltage across the capacitor which is measured by a peak voltage detector, and from this the capacitance is derived and the also the soil moisture, which is directly proportional to the capacitance.
Still another moisture sensor is described in U.S. Pat. No. 3,626,286 to Rauchwerger, in which two plates are separated by the soil, and the conductive moisture of the soil affects the capacitance which is measured between the plates. One of the problems with this type of sensor is that the soil itself is used as the dielectric material, and thus the soil composition is a factor which causes a variation in the performance of the sensor.
As revealed by a review of the prior art, existing techniques focus on the capacitance between two electrodes to determine the soil moisture. Typically, the capacitance has a low value, so that a small change in the capacitance is caused by a change in moisture, and therefore the oscillator frequency is low to allow for the reactance to be high relative to the soil resistance. The low frequency causes the capacitor to act as an electrolytic capacitor, which exhibits parasitic capacitance caused by ionic accumulation on the electrodes at low frequency, in the range below 30-70 kHz. The undesirable effect of parasitic capacitance causes problems in soil moisture measurement since the capacitance is not strictly dependent on the actual moisture, but is distorted by the parasitic capacitance field effects.
Another problem associated with the prior art moisture sensors is that they use metallic electrodes which are exposed to the soil, and so become oxidized, so that they lose their effectiveness, unless expensive metals are used.
As described above, the dominant resistance of the soil does not allow measurement of the freely available water or soil moisture, and thus the measurement is too easily affected by conductivity of the ground, which is a function of salinity.
Thus, it would be desirable to provide a soil moisture sensor which is less affected by soil resistance and provides a more accurate measurement of soil moisture than existing designs.